Recently, the application of energy storage technology has expanded to cell phones, camcorders, notebook PCs and electric vehicles, resulting in intensive research and development on batteries.
In this regard, lithium secondary batteries are one of the subjects of great interest. Particularly, according to the recent trend that electronics become downsized and lightweight, development of the lithium secondary batteries having a smaller size and a light weight and capable of charging and discharging at a high capacity has been the focus of attention.
Generally, the lithium secondary battery may be prepared using a positive electrode and a negative electrode including an electrode active material which may intercalate and deintercalate lithium ions, and an electrolyte which is a delivering medium of lithium ions. Conventionally, a liquid electrolyte, in particular, an ion conductive organic liquid electrolyte prepared by dissolving salts in a non-aqueous electrolyte solvent has been mainly used as the electrolyte.
However, there is a problem in that the liquid electrolyte may leak during operation, and ignition, explosion and the like are caused due to high flammability of the non-aqueous electrolyte solvent used. Moreover, in the liquid electrolyte, a carbonate organic solvent may decompose during charging and discharging of a lithium secondary battery, or side reactions with the electrode may be caused to generate gas in a battery. In the case of storage at a high temperature, this reaction is accelerated, leading to an increase in gas production. The gas generated continuously causes an increase in internal pressure of a battery, thereby not only causing deformation of the battery such as expansion of the thickness of the battery and the like, but also causing a local difference in the adhesion at the electrode surface in the battery, resulting in the problem that the electrode reaction does not equally occur on the entire electrode surface.
Thus, in order to overcome a safety problem of the liquid electrolyte, there has recently been proposed a method of using a polymer electrolyte which is free from leakage and the like. Examples of the polymer electrolyte include a completely solid polymer electrolyte containing no organic electrolyte and a gel polymer electrolyte containing an organic electrolyte.
The gel polymer electrolyte is prepared by impregnating a polymer matrix formed by the polymerization reaction of a polymerizable monomer and a polymerization initiator with an electrolyte containing an electrolyte salt and an electrolyte solvent, and then gelling the mixture.
The gel polymer electrolyte is superior in electrochemical stability to a conventional liquid electrolyte so that the thickness of the battery may be maintained constant, can suppress shrinkage of a separator at a high temperature by strengthening the separator, and enables the manufacture of a thin film battery because contact between the electrode and the electrolyte is excellent due to the inherent adhesion of the gel.
However, since a non-aqueous electrolyte solvent is also used in the gel polymer electrolyte, the gel polymer electrolyte has inferior battery performance as compared with the liquid electrolyte.
In order to improve the conductivity of the gel polymer electrolyte, efforts have been made to improve the conductivity of the polymer electrolyte by adding a low molecular weight polyalkylene oxide or an organic solvent as a plasticizer. However, when the content of plasticizer is increased, the physical properties of the polymer electrolyte are greatly deteriorated, or it is impossible to form a stable gel.
Research was conducted by the inventors of the present invention to overcome the above-described problems, and the research resulted in the finding of a secondary battery having high electrochemical stability and excellent battery perfolivance by introducing an isocyanate-containing monomer to a composition for a gel polymer electrolyte to form a coating layer including a urethane bond-containing oligomer on a surface of a lithium transition metal oxide.